THE GYMNOSPERMAE : CONIFERALES AND TAXALES 675 



grains separate from one another and are nourished by the absorption of 

 materials from the tapetal cells, which disintegrate as the spores ripen. 



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Microsporophyll 

 . Microsporangium 

 . Axis of cone 



Pj(j_ 674.— Pinus sylvestris. Longitudinal 

 section of the male cone showing the 

 spirally arranged microsporophylls with 

 attached sporangia. 



The microspore is at first a unicellular body covered with a three-layered 

 coat The outer laver, or extine, is heavily cuticularized, but it does no 

 completelv cover the mature spore and is found only on one side, the rest 

 o?The spore being covered by the exposed middle layer or exo-ntme. Th,s 

 layer is distended into t.o large air sacs, which greatly .ncrease the buoyancy 

 oTthe spore and aid in „s dispersal by wmd. The tnner layer or mtme >s 

 very thin (Fig. 675). 



Air sac 



Tube nucleus 



Antheridial cell 

 Inline 



Prothallial cells 

 {flattened) 



Pjq (^^^^^Pinus sylvestris. Mature microspore 

 in optical section. 



